The present invention relates to scanning of imaging plates in general and to scanning of storage phosphor medical imaging plates in particular.
(a) Image Plate Scanning:
Imaging plates, such as storage phosphor imaging plates, have become standard in the field of Computed Radiography (CR) as the medium onto which an image of a portion of the patient""s body can stored. The image on such a phosphor imaging plate is extracted by scanning the imaging plate with a scanner. Typically, a phosphor imaging plate is scanned by passing a scanning laser beam over the surface of the imaging plate while recording light emitted from the imaging plate in response to the laser beam. By recording the phosphorescence emission corresponding to each of the pixels of the imaging plate with a detector such as a photomultiplier, the image stored therein can be re-created (such that it can be displayed on a computer terminal).
The act of scanning an imaging plate by passing a scanning laser beam thereacross is inherently destructive (i.e.: it releases the energy stored in the phosphor screen). As such, a particular image stored on an imaging plate can only be scanned (i.e. read) once.
Unfortunately, when scanning an imaging plate to re-create the image stored therein (such that it can then be displayed on a computer terminal) image artifacts tend to appear in the final image. For example, alternating bands of lighter and darker regions, which run across the image, tend to be seen. As will be explained, such bands may be generated by uneven (i.e.: varying speed) movement of the imaging plate relative to the scanner (in what is commonly called the xe2x80x9cslow scan directionxe2x80x9d, and referred to herein as the xe2x80x9cYxe2x80x9d direction). This may be due to simple repeating mechanical irregularities in the scanner which thereby positions successive scan lines at uneven spacing along the length of the imaging plate. It may also be caused by vibrations perpendicular to the plane of the imaging plate which affect the optical focus of the scanning mechanism. In addition, various multi-head scanning systems tend to generate artifacts simply due to the fact that the different scanning heads each have their own optical paths which exhibit different optical characteristics. This is especially true in the case where each of the various scanning heads has its own dedicated laser.
Therefore, unwanted image artifacts can be divided into two broad groups. The first being those unwanted image artifacts caused by variations in the speed of movement of the scanner with respect to the imaging plate or by small vibrations either in the slow scan (i.e.: xe2x80x9cYxe2x80x9d) direction or normal to the imaging plate. The second being those unwanted image artifacts caused by differences between various scanning heads (when using a scanner with more than one scanning head). These two groups are discussed separately below.
(b) xe2x80x9cRipplexe2x80x9d or xe2x80x9cBandingxe2x80x9d Artifacts:
A variety of different systems exist to scan imaging plates, such as storage phosphor imaging plates by passing one or more scanning heads over the surface of the imaging plate.
In a first existing system, a single scanning head is moved back and forth across the surface of the imaging plate while the imaging plate is moved relative to the scanner in the Y direction. Specifically, the imaging plate is moved in a direction that is perpendicular to scanner head movement such that the scanning head passes over the imaging plate along a plurality of parallel or generally parallel paths (in an xe2x80x9cXxe2x80x9d direction). In one type of system, a rotating or oscillating mirror directs a laser beam across the imaging plate, and the imaging plate is then advanced an incremental distance. This process is repeated such that the scanning head traces a series of parallel paths across the imaging plate. In another type of system, the imaging plate is continuously advanced as the scanning head is passed thereover, such that the scanning head traces a series of parallel paths across the imaging plate. Alternatively, the scanning head may itself be moved back and forth in the X direction across the surface of the imaging plate.
In a second existing system, the imaging plate is wrapped around a cylinder, and the cylinder is rotated while a single scanning head moves down the length of the cylinder. An example of such a system is found U.S. Pat. No. 5,635,728.
In a third system, which is novel and was developed by the present Applicants, a plurality of (typically three) scanning heads are positioned around the perimeter of a rotary scanner, and the scanner is rotated while an imaging plate is advanced thereunder. An example of such a system is found in PCT Published Application WO 00/19477. In this system, each of the scanning heads sequentially trace a curved path across the surface of the imaging plate and the movement of the imaging plate thereunder causes these curved paths to be spaced apart from one another along the length of the imaging plate. As the imaging plate is advanced under the rotating scanner, the entire surface of the plate is scanned.
Unfortunately, in all of the above described systems, any inconsistency or periodic variation in the speed of movement between the imaging plate and the scanner will result in successive scan lines (i.e.: the paths taken by the scanning head(s) across the surface of the imaging plate) being spaced unevenly apart. This unevenness between successive scan lines causes xe2x80x9cbandingxe2x80x9d or xe2x80x9cripplesxe2x80x9d to occur in the final image. This is true both in the case of a linear path scanner which is kept at a fixed location with its scanning head directing a laser beam in a straight path across an imaging plate, and in the case where a plurality of scanning heads are rotating around a common center of a scanner.
As mentioned above, the scanning of an imaging plate releases the energy trapped therein. Therefore, when successive scan lines are too close together, the edges of the laser beam spot (which passes along each successive scan line) will tend to overlap such that xe2x80x9coversamplingxe2x80x9d of the image occurs. In other words, part of the energy representing the brightness of the image stored in a particular pixel will have already been released by the previous scan line, thereby reducing the intensity of the image when the pixel is scanned. As such, the image energy trapped within a second pixel disposed on a second scan line will have been partially released when a first (ie: previous) adjacent scan line has passed over the imaging plate. When a region of the imaging plate has been oversampled in this manner, a dark band will tend to occur which runs across the image (in a path generally parallel to the scan lines). Conversely, should the successive scan lines be positioned too far apart, the image will tend to be undersampled, resulting in a light band passing across the image.
Even a very small degree of unevenness in the scan line spacing can give rise to detectable banding artifacts in this type of scanner because the pixel intensities are preferably digitized to a high degree of precision (typically 16 or more bits per pixel).
Such alternating light and dark bands will become especially apparent when the intensities of the individual pixels in the image are scaled and presented to an operator in a final (on screen or printed) image. Such alternating banding will typically appear as thin bands in the final (on screen or printed) image such that the image appears to have xe2x80x9cripplesxe2x80x9d running along its length. In the case of a linear back and forth scanning head, these ripples will appear as straight lines and in the case of a rotary scanner, these ripples will appear in curved arcs.
The unevenness in the speed at which the imaging plate moves relative to the scanner is typically introduced by very small mechanical inaccuracies in the transportation system that moves the imaging plate. For example, should movement of the imaging plate be performed by a transport mechanism which comprises a worm gear, the center worm gear may itself be at least slightly off-axis. In this case, rotation of the worm gear at a constant angular speed results in a repeating pattern of variable speed changes in the movement of the imaging plate. Specifically, this pattern (which may comprise the movement of the imaging plate continually changing speed to different speeds) will repeat once for every rotation of the worm gear.
There are many alternate drive configurations that may be employed in the slow scan (i.e.: xe2x80x9cY-directionxe2x80x9d) transport mechanism that can give rise to small periodic velocity variations due to mechanical tolerance limitations. Examples include gear trains and pulleys. Accordingly, since several different factors may introduce speed variances at the same time, the periodic pattern of lighter and darker bands in the image may have components at different frequencies. Harmonics of a fundamental frequency may also occur due to the particular characteristics of a periodic vibration source.
Accordingly, what is desired is a system which both detects, and compensates for, repeating patterns of variations in scan line distance separation along the length of the imaging plate, such that xe2x80x9cbandingxe2x80x9d or a xe2x80x9crippledxe2x80x9d appearance of the final (on screen or printed) image can be avoided.
(c) Multiple Scanning Head Artifacts:
An important advantage of multi-head scanning systems which pass a plurality of separate scanning heads across an imaging plate is that they can increase both the speed and duty cycle of the scanning. A disadvantage of such multi-head scanning systems is that each of the scanning heads will typically have a different detective gain. Accordingly, each scanning head will read a slightly different image intensity (i.e.: detect a slightly different signal) for the same amount of actual phosphorescence emissions signal actually received therein.
Although such differences in detective gain vary among the various scanning heads (i.e.: at spaced apart scan lines in the Y-direction), such differences in detective gain may also vary among the various scanning heads depending upon the position of the scanning head across the imaging plate (i.e.: such differences in detective gain may also vary in the X-direction).
This is due to the fact that each scanning head has its own optical train, which will have its own light transmission characteristics. Furthermore, should each of the separate scanning heads/optical trains have its own dedicated laser, differences in laser output strength among the various lasers will also occur. In addition, each of the scanning heads may tend to focus their laser beams at slightly different locations. For example, in the case of a rotating multi-head scanner with a plurality of scanning heads located around its perimeter, each of the scanning heads may tend to focus its laser beam at slightly different radial distance from the center of the scanner. Accordingly, when such a scanner is rotated (at a x fixed location positioned over a constantly moving imaging plate) the successive scan lines across the imaging plate will tend to be somewhat unevenly spaced apart. Therefore, such multiple scanning head image artifacts can therefore exhibit a repeating pattern in the Y-direction, constituting yet another form of the above discussed xe2x80x9cripplexe2x80x9d artifacts. As also noted in the above discussion of ripple artifacts, even very small irregularities in scanning head spacing can give rise to detectable gain artifacts.
Moreover, in addition to the above average or overall differences in detective gain occurring among the various scanning heads (i.e. in which at least one scanning head reads an image to be somewhat lighter or darker than another scanning head, for reasons explained above), a further complication may exist for rotary scanners.
Specifically, for each of the multiple scanning heads in a rotary scanner, the average detective gain will also tend to vary depending upon the radial position of the individual scanning head as the scanning head moves across the imaging plate. This is especially true when the rotary scanner comprises a single stationary photodetector at its center with each scanning head directing phosphorescence emissions back to the centrally located photodetector. In such systems, each of the separate laser beams will rotate on the surface of the photodetector as the scanning head moves across the surface of the imaging plate.
Therefore, when using a multi-head rotary scanner, individual differences in detective gain will exist among the various scanning heads and these differences will also change depending upon the radial position of the scanning head as it moves in a curved path across the imaging plate. Furthermore, such characteristic variations will tend to be unique to each scanning head.
Accordingly, what is desired is a system which both detects, and compensates for, overall variances in detective gain among various different scanning heads, and also compensates for such variances in detective gain among the various scanning heads depending upon the radial position of the scanning head. Such a system would therefore compensate for signal variances in both the X (across the imaging plate) and Y (along the imaging plate) directions.
The present invention provides a variety of methods and systems for detecting and compensating for repeating patterns of variations in scan line distance separation along the length of the imaging plate (i.e.: in the Y-direction), such that xe2x80x9cbandingxe2x80x9d or a xe2x80x9crippledxe2x80x9d appearance of the final image can be avoided. In addition, the present invention provides a variety of methods and systems for detecting and compensating for overall variances in detective gain among various different scanning heads, and also compensates for such variances in detective gain among the various scanning heads depending upon the radial position of the scanning head. Thus, the present system advantageously compensates for signal variances in both the X (across the imaging plate) and Y (along the imaging plate) directions.
In a preferred aspect, the present invention provides a method of compensating for differences in detective gain between a plurality of different scanning heads in a multiple scanning head imaging plate scanner, comprising: (a) scanning each of the scanning heads across an imaging plate thereby determining the detected signal at successive locations across the imaging plate for each of the scanning heads; (b) calculating an inverse relationship to the detected signal at successive locations across the imaging plate for each of the scanning heads; (c) scanning each of the scanning heads across an imaging plate containing an image thereon, thereby determining an image value at the successive locations across the imaging plate for each of the scanning heads; and (d) applying the inverse relationship to the determined image values at the successive locations across the imaging plate for each of the scanning heads.
In alternative aspects, the present invention provides a method of compensating for non-uniformity effects in a rotary scanner, comprising: (a) scanning at least one scanning head across an imaging plate thereby determining the detected signal at successive locations across the imaging plate; (b) calculating an inverse relationship to the detected signal at successive locations across the imaging plate; (c) scanning the at least one scanning head across an imaging plate containing an image thereon, thereby determining an image value at the successive locations across the imaging plate; and (d) applying the inverse relationship to the determined image values at the successive locations across the imaging plate.
In alternative aspects, the present invention provides a method of detecting periodic variances in signal values produced by scanning an exposed imaging plate with an imaging plate scanner having at least one scanning head, comprising: (a) moving the exposed imaging plate relative to the imaging plate scanner while repetitively scanning across the imaging plate with the at least one scanning head such that the at least one scanning head scans across the imaging plate in a series of scan lines which are spaced apart along the length of the blank imaging plate; (b) scanning the at least one scanning head across the imaging plate, thereby measuring a detected signal at successive locations along each scan line in the series of spaced apart scan lines; (c) calculating a signal value representative of each of the scan lines in the series of scan lines; and (d) identifying a repeating pattern in the signal values representative of each scan line in the series of spaced apart scan lines.
In alternative aspects, the present invention provides a method of compensating for image artifacts caused by the periodic variances in signal values produced by scanning an exposed imaging plate with an imaging plate scanner having at least one scanning head, comprising: (a) calculating a correction transfer function corresponding to a repeating pattern of the periodic variances in the signal values; (b) scanning the at least one scanning head across an imaging plate containing an image thereon, thereby determining an image value at successive locations across the imaging plate for each scan line in the series of scan lines; and (c) applying the correction transfer function to the determined image values at the successive locations along each of the scan lines passing across the imaging plate.
In alternative aspects, the present invention provides a method of compensating for image artifacts caused by the periodic variances in signal values produced by scanning an exposed imaging plate with an imaging plate scanner having at least one scanning head, comprising: (a) calculating a correction transfer function corresponding to the repeating pattern in the signal values; (b) scanning the at least one scanning head across an imaging plate containing an image thereon, thereby determining an image value at successive locations across the imaging plate for each scan line in the series of scan lines; and (c) varying the speed of relative movement between the imaging plate and the imaging plate scanner in accordance with the correction transfer function.